The aim of this work is the study of the S retention in ashes from combustion of three bituminous coals and a sewage sludge, processed with lime front an urban wastewater treatment plant, that uses FeCl3 as a coagulant. The effect of the sludge addition during co-combustion of sludge-coal blends in the proportions 10 and 50 wt% of sludge is analysed. The combustion was carried out in an electric furnace at different temperatures (800. 900, and 1100 degreesC). The results have confirmed that sludge addition to coal enhances S retention at any temperature due to the high CaO contents of the sludge and the formation of CaSO4. However, the addition of FeCl3 may be prejudicial, since it produces Na and K volatilisation during cocombustion. Moreover, the presence of Fe2O3 from FeCl3 can reduce the amount of retained sulphur due to its reaction with CaO to produce calcium ferrite. Another element with a similar effect to Fe is Si, since it forms larnite (2CaO.SiO,) in coal-sludge blends with high Si and Ca contents. Lineal relationships have been found between S retention and the above oxide contents. (C) 2004 Elsevier Ltd. All rights reserved.